1. Technical Field
This invention relates generally to a system and method for determining knock in an ion current sensing system of the type useful in connection with controlling ignition in an internal combustion engine.
2. Description of the Related Art
One approach for detecting a combustion condition, such as knock or misfire, involves the use of a so-called ion sense system. It is known that the combustion of an air/fuel mixture in an engine results in molecules in the cylinder being ionized. It is further known to apply a relatively high voltage across, for example, the electrodes of a spark plug just after ignition in order to produce a current between the electrodes. Such current is known as an ion current. The ion current that flows is, generally speaking, proportional to the number of combustion ions present in the area of, for example, the spark plug gap referred to above. Additionally, the level of such ion current may provide some measure of the level of ionization throughout the entire cylinder as combustion occurs. The DC level or amount of ion current is indicative of the quality of the combustion event, or whether in fact combustion has occurred at all (e.g., a misfire condition). An AC component of the ion current may be processed to determine the presence of knock.
It is also known to determine knock which involves processing a signal from a knock sensor only during a so-called knock window, as seen by reference to U.S. Pat. No. 5,537,855 issued to Hunninghaus et al. entitled xe2x80x9cKNOCK DETECTION METHOD AND APPARATUS WITH DUAL INTEGRATION WINDOWS.xe2x80x9dHunninghaus et al. discloses a knock sensor having an output that is processed during dual knock windows. The knock signal from the sensor is processed during one angular window or period (i.e., produced by a controller based on a crank position sensor) at a point in an engine combustion cycle where a knock event is expected to occur. A shortcoming of this approach, whether using a conventional knock sensor or using an ion sense system, is that such a knock window constitutes, at best, only a prediction by the controller as to when the knock is going to occur. There is therefore a margin for error with controller-generated knock windows, which may permit processing of information that may be mistaken for knock (e.g., noise), resulting in a false indication of knock by the knock detection system.
U.S. Pat No. 6,089,077 issued to Daniels entitled xe2x80x9cMASS FRACTION BURNED AND PRESSURE ESTIMATION THROUGH SPARK PLUG ION SENSINGxe2x80x9d discloses an ion sensing system. The system of Daniels further discloses determining an inflection point in the ion signal and calculating a mass fraction on that basis.
There is therefore a need to provide an improved apparatus for detecting a combustion condition, such as knock, that minimizes or eliminates one or more of the shortcomings as set forth above.
One object of the present invention is to provide a solution to one or more of the above-identified problems. One advantage of the present invention is that it permits internal generation of a knock window (i.e., internal to an ignition coil configured with the invention), thereby eliminating the need for an engine controller or the like to generate a knock window, with all its disadvantages as described in the Background. Another advantage is that the internally generated knock window is superior to a controller-generated knock window inasmuch as it is generated based on real data, not a guess based on crank angular position. This improvement yields a more accurate knock intensity signal. In an alternate embodiment, the knock signal (e.g., indicative of knock intensity) is sent to the engine controller over an ignition control signal line (i.e., the same line that carries the signal for controlling dwell and spark timing to the ignition coil). This multiplexing of the two signals on one physical line allows for the elimination of one wire from the overall system, decreasing cost and improving reliability.
These and other objects, features and advantages are realized by a method and apparatus according to the invention that operates based on detection of the slope of the ion current signal. The knock signal occurs on the last decreasing voltage portion of the ion current signal waveform for each ignition event. Processing the ion current signal for knock only during this time (i.e., the defined knock window) will reduce much of the noise that may be mistaken for knock in prior, conventional systems.
An apparatus according to the present invention includes an ion current detection circuit, a knock detector, and a slope detector. The ion current detection circuit is configured to bias a spark plug in an engine cylinder for producing an ion current signal indicative of a level of combustion in the cylinder. The ion current signal includes knock components when knock is present in the cylinder. The knock detector is enabled during a knock window for generating a knock signal that is indicative of knock in the cylinder, based on the ion current signal. The slope detector is configured to enable the knock detector when the ion current signal exhibits a predetermined slope indicative of a time interval over which knock is to occur.
A method of detecting knock is also presented.